1. Field of the Invention
The present invention relates to circuitry for providing compression of uncompressed analog PAM signals and expansion of compressed analog PAM signals and more particularly to those instantaneous compressors and expandors which utilize the logarithmic characteristics of diodes to provide such compression and expansion.
2. Description of the Prior Art
In PCM (pulse code modulation) communication systems, continuous time varying information signals, such as electrical speech signals, may be represented by a series of on and off pulses. At a transmitting station the signal is periodically sampled, quantized, and encoded into binary code words which are indicative of the amplitude of the signal. The binary coded signals are then transmitted over a medium such as a telephone transmission line to a receiving station. At the receiving station the binary code words are decoded and reconverted back to continuous time varying information signals.
When the signal is quantized for transmission the difference between the instantaneous value of the signal and the quantum level actually transmitted gives rise to what is known as quantizing noise or quantizing distortion. This distortion is especially objectionable when the instantaneous value or magnitude of the input signal is small, but is usually of little or no significance when the instantaneous value of the signal is high. To reduce this distortion it is therefore desirable to have a nonlinear redistribution of the total number of quantizing levels available. This nonlinear redistribution is called "companding" which is a verbal contraction of the terms "compression" and "expansion."
At the transmitter companding is provided by a compression circuit which has a known compression characteristic. At the receiver the received companded signal is expanded by an expandor circuit having an expansion characteristic which is complementary to the compression characteristic to thereby provide overall linearity in the PCM system.
A current practice in the quantization of analog signals for transmission in telephone system applications is to encode the input signal logarithmically according to a "mu-law" companding scheme. Logarithmic mu-law encoding of signals provides reasonably constant signal to quantization noise levels over a wide dynamic range of input voice signals. Thus, logarithmic encoding is desirable for speech processing.
The mu-law encoding characteristic provides a smooth logarithmic curve over the range of input signal amplitude. The characteristic is usually implemented by circuitry which provides a piecewise linear approximate equivalent to the logarithmic characteristic. For example, a mu-225 code implies an encoding scheme that employs 8 positive and 8 negative segments (chords) to approximate the logarithmic characteristic. In practice, this is a 15 chord approximation law since the inner positive and innermost negative chords are colinear.
There have been many attempts in the prior art to provide circuitry for compressors and expandors which approximates the mu-law logarithmic characteristic. One such compandor circuit is described in U.S. Pat. No. 3,176,224 which issued on Mar. 30, 1965. The compandor circuit described therein utilizes ramp or slope functions which are generated or derived from, and in time sequence, with, the cyclic pulse repetition rate of the PCM system. The compression and expansion characteristics provided are parabolic rather than the ideal logarithmic configuration. This approach is utilized in order that the problems associated with earlier compressors and expandors which utilize the non-linear characteristics of diodes are eliminated. Compandors utilizing the nonlinear characteristics of diodes exhibited signal degradation due to nonuniformity of the temperature stability properties and/or the characteristic curves of the diodes used at each end of the transmission circuit. Among the techniques used in the prior art to provide temperature stability are the placement of the diodes in ovens designed to provide a substantially constant diode operating temperature.
Another attempt to provide an encoder for the mu-law characteristic is described in U.S. Pat. No. 3,905,028 which issued on Sept. 9, 1975. The encoder described therein utilizes a minimal amount of analog circuitry in conjunction with digital components to directly convert analog signals into a code exhibiting logarithmic characteristics. Still more recent attempts to provide encoders and decoders having logarithmic characteristics has resulted in the development of integrated circuits known as codecs which is a verbal contraction of the terms "encoder" and "decoder." A typical example of such a codec are the models DF 331 coder and DF 332 decoder both of which are manufactured by Siliconix of Santa Clara, Calif.
Present multichannel PCM Carrier for use in telephone systems is usually designed to meet a standard which specifies the number of bits to be transmitted per second and the type of companding characteristic and number of bits included therein. One such type of system is the T.sub.1 D.sub.3 system which has been put into effect by the American Telephone and Telegraph Company in their associated Bell operating companies. In this system 24 channels of binary coded information are transmitted at the rate of 1.544.times.10.sup.6 bits per second. This system utilizes an 8 bit mu-255 companding characteristic. Each of the 24 channels are periodically sampled at an 8 kilohertz rate and the resultant samples are then quantized and encoded.
Codecs could be used in the above system to perform the quantizing and encoding steps at the transmitter and the decoding and conversion steps at the receiver. The basic building block of any codec is a companding analog to digital converter. There are, however, no companding analog to digital converters and thus no codecs which are fast enough to handle the encoding of all 24 channels by a single codec. The alternatives then are to utilize either one codec per channel or have a limited number of channels such as eight (8) share a codec. Neither of these approaches provide a cost effective solution to the problem of encoding and decoding 24 channels in a T.sub.1 D.sub.3 PCM system.
It was then recognized that it would be far more desirable to provide a single analog compandor for the compression and expansion of the 24 channels. It was further recognized that such a compandor could be simply and cost effectively provided utilizing the logarithmic characteristics of diodes in order to provide an approximation to the mu-255 characteristic. The analog compandor of the present invention in conjunction with linear encoding after the compressor and linear decoding before the expandor provides the approximation to the mu-255 characteristic through the use of the logarithmic characteristics of diodes. The compandor of the present invention also substantially eliminates the problems associated with earlier compressors and expandors which used the nonlinear characteristic of diodes to approximate the companding characteristic.